Apply Flutter best practices [Non-Breaking] (#2)

* Add flutter_lints package

* Use key in widget constructors

lint: use_key_in_widget_constructors

* Unnecessary new keyword

lint: unnecessary_new

* Prefer const with constant constructors

* Exclude protobuf files from analyzer

* Private field could be final

* Annotate overridden members

* Use initializing formals when possible

* Don't access members with `this` unless avoiding shadowing

* Prefer is! operator

* Use isEmpty instead of length

* Use collection literals when possible

* Revert comment changes

* Cleaner null testing

* Misc fixes

* Avoid using `forEach` with a function literal

* Prefer `final` over `var` where applicable

* Enforce stricter type-checking

* Ignore VS Code files

* Flutter format

* Prefer using lowerCamelCase for constant names

* flutter format -l 100

* Disable `avoid_print` for examples

* Slight improvements to example

Should solve lint error: no_logic_in_create_state

lib/src/livekit.dart

@@ -5,9 +5,8 @@ import 'options.dart';
 /// {@category Room}
 class LiveKitClient {
   /// Connects to a LiveKit room
-  static Future<Room> connect(String url, String token,
-      [JoinOptions? options]) {
-    var room = Room();
+  static Future<Room> connect(String url, String token, [JoinOptions? options]) {
+    final room = Room();
     return room.connect(url, token, options);
   }
 }